In semiconductor fabrication processes, the resolution of a photoresist pattern begins to blur at about 45 nanometer (nm) half pitch due to diffraction. To continue to use fabrication equipment purchased for larger technology nodes, multi-exposure methods have been developed.
Multiple exposure or multi-patterning technology (MPT) involves forming patterns on a single layer of a substrate using two or more different masks in succession. If only two masks are used for patterning a layer, the technique is referred to as double exposure. One form of double exposure is referred to as double patterning technology (DPT). In DPT, first and second masks are used sequentially to pattern the same layer. As long as the patterns within each mask comply with the relevant minimum separation distances for the technology node, the combination of patterns formed using both masks may include smaller separations than the minimum separation distance. MPT allows line segments, and in some cases, more complex shapes to be formed of a vertical segment and a horizontal segment on the same mask. Thus, MPT provides flexibility and generally allows for significant reduction in overall IC layout.
MPT is a layout splitting method analogous to an M-coloring problem for layout splitting in graph theory, where M is the number of masks used to expose a single layer (and M also equals the number of exposures of photoresist over the single layer of the substrate, where each exposure is performed using a respectively different mask). For example, if two masks are to be used (double patterning, DPT), it is customary to refer to the patterns as being assigned one of two “color types”, where the color corresponds to a photomask assignment.
An MPT native conflict exists for a given number of photomasks, when there is no way to assign all the circuit patterns to the given number of photomasks, without violating minimum separation design rules. There will be at least two circuit patterns assigned to the same one of the photomasks, such that those two circuit patterns remain closer to each other than the minimum separation distance determined by design rules for a given technology node. For example, in the case of MPT with three photomasks for exposing a single layer (triple patterning technology, TPT), there is a triple patterning native conflict if there is no way to divide the circuit patterns of that layer among three masks without violating the minimum separation distance design rule in at least one of the three masks. The designer can resolve the MPT conflict without increasing the number of masks by making a change to the layout.